As discussed on this sage-devel thread, we propose to move the Cython code that used to be in the optional package giacpy_sage into sage/libs/giac/.

Depends on #30277

CC: @frederichan-IMJPRG @mwageringel @slel @kiwifb

Component: interfaces

Author: Vincent Delecroix, Matthias Koeppe, Frederic Han

Branch/Commit: 911ddfb

Reviewer: Matthias Koeppe, Frederic Han, Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/29171